The present invention relates to solid state ignition systems and, more particularly, to an adaptive dwell ignition system wherein the excess dwell time is varied with engine rpm such that the excess dwell period is a constant percent time.
Adaptive dwell ignition systems are well known to those skilled in the art. For example, U.S. Pat. No. 4,117,819 discloses an adaptive dwell ignition system to which the subject invention is an improvement thereover. The ignition system of the aforereferenced patent comprises an adaptive dwell capacitor which is charged and discharged during a firing cycle. The capacitor is connected to one end of a sensor coil, at a first input of the ignition system, through a buffer amplifier of the ignition system. The other end of the sensor coil is connected to a second input of the ignition system. The second end of the sensor coil is also coupled to ground reference potential through a resistor divider circuit.
Briefly, timing signals are developed across the sensor coil in timed relationship to operation of the engine. The timing signals are superimposed onto the voltage appearing at the first end of the sensor coil, which voltage is essentially the voltage developed across the adaptive dwell capacitor. In response to the voltage developed at the second end of the sensor coil exceeding a reference potential the ignition system produces current to charge the primary of the ignition coil during the firing cycle. At a predetermined value the charging current is limited by regulating means of the ignition system. The portion of the firing cycle period during which the current is limited is typically referred to as the excess dwell time. At the end of the firing cycle the coil current is abruptly terminated which causes the field to collapse about the coil. This action produces spark to drive the engine.
In order to minimize power dissipation and to prevent mis-spark or no-spark conditions it is desirable to minimize the excess dwell time while ensuring adequate field energy. Thus, as the engine rpm is varied the time at which coil current is initiated in a firing cycle is varied. The manner in which this is done is by charging and discharging the adaptive dwell capacitor accordingly. This causes the reference level upon which the timing signal rides to change which in turn causes the coil current to be turned on either earlier or later in the cycle as is understood.
A problem with this prior art system is that the adaptive dwell capacitor is charged through the external resistor divider circuit whereas the discharge thereof is through a current source which is internal to the ignition system which is fabricated as a monolithic integrated circuit. Therefore, due to temperature and process variations, the ratio between the charge and discharge currents can vary. This will cause the excess dwell time at any particular engine rpm to vary which is undesirable.
Additionally, the external resistor divider circuit requires a third input to the integrated circuit. This means that an additional pin out is required.
Furthermore, the resistor divider circuit has closed loop currents flowing therethrough which necessarily requires small valued resistors to be used to maintain proper charging of the adaptive dwell capacitor but which limits protection of the integrated circuit to inductively and capacitively coupled voltage transients that may occur across the sensor coil.
In view of the above, it is desirous to have an adaptive dwell ignition system of the type described in which the excess dwell current is maintained a constant percent of time and is also independent of temperature and process variations. Moreover, it is desirable to eliminate as many pin out requirements for the integrated circuit as possible for cost reductions. Still further, it is desirable to eliminate external loop currents and to provide voltage transient protection to the ignition system.